1. Field of the Invention
The present invention generally relates to an ATM (Asynchronous Transfer Mode)/STM (Synchronous Transfer Mode) coexistence network system and more particularly to an interface apparatus of the ATM/STM coexistence network system.
2. Description of the Related Art
In the existing telephone-service exchange network in North America which network provides the existing narrow-band digital optical subscriber market mainly with telephone services, digital loop carrier systems are installed at the exchange and at the subscriber's home. By virtue of an ATM exchange provided in such an area, the demand for installation of subscriber apparatuses for providing broad-band transmission services is rising vary fast.
In the existing system, the STM communication method which is entirely different from the ATM communication method is adopted. In the ATM/STM coexistence network system provided by the present invention, the STM method is mixed with the multiplexing/demultiplexing ATM method serving as a means for carrying out batch transmission of services having different frequency bands. In this way, services can be provided to the subscriber's home by using both the methods with the highest degree of efficiency without the need to modify the existing facilities.
Traditionally, the conventional synchronous system adopting the STM method is used for providing telephone services. As shown in FIG. 16, the conventional system which is used for providing telephone services comprises an existing telephone-service apparatus 1 and a telephone set 2 at the subscriber's home. The telephone set 2 is connected to the existing telephone-service apparatus 1 by means of a synchronous clock signal through an NBS (Narrow-Band Service) supplying apparatus 3 which is shown in detail in FIG. 17. The existing telephone-service apparatus 1 shown in FIG. 16 comprises an STM exchange 4, a central office terminal 5 connected to the STM exchange 4 and a CMS (Common-Shelf) apparatus 6, a remote terminal which has an audio-signal processing function such as a DS0 cross connect and SONET (Synchronous Optical Network) multiplexing/demultiplexing of the telephone-service system.
As shown in FIG. 17, the NBS supplying apparatus 3 connected between the CMS apparatus 6 and the telephone set 2 comprises an STS-1 transmission receiving circuit 8, a synchronous format conversion circuit 9, a multiplexing conversion/framer circuit 10 and an analog-signal D/A (Digital-to-Analog) conversion circuit 11 which are connected to the CMS apparatus 6 in series in a direction from the CMS apparatus 6 to the telephone set 2 referred to hereinafter as the downward direction. In addition, the NBS supplying apparatus 3 also includes an audio-signal A/D (Analog-to-Digital) conversion circuit 12, a multiplexing inverse conversion/deframer circuit 13, a synchronous format conversion circuit 14 and an STS-1 transmission transmitting circuit 15 which are connected to the telephone set 2 in series in a direction from the telephone set 2 to the CMS apparatus 6 referred to hereinafter as the upward direction.
In the downward direction in the configuration described above, a DS0 signal for 192 subscribers transmitted by the CMS apparatus 6 is received by the STS-1 transmission receiving circuit 8 in an STS-1 exclusive frame format. It should be noted that the DS0 is an abbreviation for the Digital Signal - Level 0, a signal in a bandwidth of 64 Kbps conforming to the TR-008 standard of North America. The STS-1 stands for the Synchronous Transport Signal - Level 1 Concatenation, one of formats in a bandwidth of 51.84 Mbps conforming to SONET of North America.
The STS-1 signal received by the STS-1 transmission receiving circuit 8 is converted by the synchronous format conversion circuit 9 into a signal having a frame format specially synchronized to the system clock. The signal in the converted frame format is split by the multiplexing conversion/framer circuit 10 into a DS0 signal allocated to a specific subscriber and communication signaling information for the STM exchange 4. The audio-signal D/A conversion circuit 11 converts the digital DS0 signal from a digital signal into an analog audio-band signal which is then transmitted to the telephone set 2.
In the upward direction, on the other hand, an analog audio-band signal transmitted by the telephone set 2 is converted into a digital DS0 signal by the audio-signal A/D conversion circuit 12. The DS0 signal is multiplexed with communication signaling information of the STM exchange 4 into a signal having a special frame format by the multiplexing inverse conversion/deframer circuit 13. The signal in the special frame format is converted by the synchronous format conversion circuit 14 into an STS-1 signal which is then transmitted to the CMS apparatus 6 through the STS-1 transmission transmitting circuit 15.
As described above, in the STM exchange network system, signal processing and transmission of information by means of a synchronous signal are carried out in the system components, from the STM exchange 4 to the NBS apparatus 3. A band is allocated to each of telephone sets 2 installed in the subscribers' homes in advance. The bands are multiplexed in a special frame format in a signal conforming to SONET.
By the way, an ATM method can be introduced into the STM exchange network system described above in order to build an ATM exchange network shown in FIG. 16 by providing an ATM exchange 17, an HDT (Host Digital Terminal) station 18 connected to the ATM exchange 17 by an optical fiber, a BAS (Broad-band Access Shelf) apparatus 19 connected to the HDT station 18 by an optical fiber and STB (Set Top Box) apparatus 20 connected to the BAS apparatus 19 by a coaxial cable. Installed in the subscriber's home, the STB apparatus 20 terminates an ATM cell and has a function for compressing cell information. Terminal equipment such as the telephone set 2 and a video apparatus which is not shown in the figure is connected to the STB apparatus 20.
In the ATM exchange network described above, after being processed by the ATM exchange 17 and received by the HDT station 18, ATM cells are transmitted to the BAS apparatus 19. Then, in the BAS apparatus 19, coaxial cables which are bus-connected to the STB apparatuses 20 belonging to a plurality of subscribers transmit the ATM cells to the subscribers in order to provide telephone services thereto.
By the way, even though the ATM exchange network described above is superior in that signals of different bands and different communication services can be provided effectively with a high degree of quality, the ATM exchange network has characteristics that are not compatible with the STM exchange network built as a synchronous system like the existing telephone service system. If an apparatus which allows the two networks with different methods to be integrated into one is developed, the cost to modify the existing system and the cost to develop the apparatus itself will be high. On the top of that, there will also be a problem of poor operatability in addition to the high-cost problem.